16. a4b5  16... a6b7 Black's moves after the knight sac were all forced. Now, White has a difficult choice. 17. e6d4 This move, I think, took Warmerdam by surprise since he thought for more than 40 minutes on his reply. 17... f6f7 A good move, preventing the threat of e4-e5+. 18. a1a4 Threatening the bishop on b4.